A progressive cavity pump, comprising a rotor and a stator, transfers fluid by means of a sequence of discrete cavities that move through the pump as the rotor is turned within the stator. Transfer of fluid in this manner results in a volumetric flow rate proportional to the rotational speed of the rotor within the stator, and relatively low levels of shearing applied to the fluid. Hence, progressive cavity pumps have typically been used in fluid metering and pumping of viscous or shear sensitive fluids.
A progressive cavity pump (PCP) may be used in reverse as a positive displacement motor to convert the hydraulic energy of a high pressure fluid into mechanical energy in the form of speed and torque output, which may be harnessed for a variety of applications, including downhole drilling. A positive displacement motor (PDM) comprises a power section including a rotor disposed within a stator, a bearing assembly, and a driveshaft. The driveshaft is coupled to the rotor of the power section and supported by the bearing assembly. Fluid is pumped under pressure through the power section, causing the rotor to rotate relative to the stator, thereby rotating the coupled driveshaft. In general, the rotor has a rotational speed proportional to the volumetric flow rate of fluid passing through the power-section. Another component, for example, a drill bit for downhole drilling, may be attached to the driveshaft. As high pressure fluid is pumped through the power section, rotary motion is transferred from the rotor to the drill bit through the bearing assembly and driveshaft, permitting the rotor to turn the drill bit.
A PCP or power section of a PDM generally includes a helical-shaped rotor, typically made of steel that may be chrome-plated or coated for wear and/or corrosion resistance, and a stator, typically a heat-treated steel tube lined with a helical-shaped elastomeric insert. FIG. 1 illustrates a perspective, cut-away view of a conventional rotor-stator assembly 5 comprising a rotor 10 disposed within a stator 20. This rotor-stator assembly 5 may be employed as a PCP or the power section of a PDM. FIG. 2 illustrates a cross-sectional view of the conventional rotor-stator assembly 5 depicted in FIG. 1. As shown in this figure, the rotor 10 has one fewer lobe 15 than the stator 20. When the two components are assembled, a series of cavities 25 are formed between the outer surface 30 of the rotor 10 and the inner surface 35 of the stator 20. Each cavity 25 is sealed from adjacent cavities by seal lines formed along the contact line between the rotor 10 and the stator 20. The center 40 of the rotor 10 is offset from the center 45 of the stator 20 by a fixed value known as the “eccentricity” of the rotor-stator assembly 5.
During operation of a PDM, high pressure fluid is pumped into one end of the power section where it fills the first set of open cavities. The pressure differential across the two adjacent cavities forces the rotor to turn. As previously stated, a PCP may be described as operating in reverse of a PDM, meaning the application of speed and torque to the PCP rotor causes the rotor to rotate within the stator, resulting in fluid flow through the length of the PCP, whereas fluid flow through the power section of a PDM causes the rotor to turn. In both types of assemblies, adjacent cavities are opened and filled with fluid as the rotor turns. As this rotation and filling process repeats in a continuous manner, fluid flows progressively down the length of the PCP or the power section of the PDM. Moreover, as the rotor turns inside the stator, the rotor's center moves in a circular motion about the stator's center. Because the rotor center is offset from the stator center, out of balance forces are generated by the rotation or nutation of the rotor within the stator. Without being limited by theory, it is believed that the greater the eccentricity of the PCP or power section of the PDM, the higher these out of balance or centrifugal forces.
Rotor-stator assembly failures may occur due to the destruction of the stator elastomer. Mechanical failure of the elastomer occurs when it is overloaded beyond its stress and strain limits, such as may be caused by a high compression fit between the rotor and stator. Thermal failure of the elastomer occurs when the temperature of the elastomer exceeds its rated temperature for a prolonged period. Even for shorter periods of time, increasing elastomer temperature causes elastomer physical properties to weaken, resulting in a shortened elastomer life.
There are several mechanisms or modes of heat generation that may elevate the elastomer temperature above its rated temperature as follows: interference, hysteresis, centrifugal forces, and downhole sources. Interference between the rotor and the stator is necessary to seal the discrete cavities. Centrifugal forces are exerted on the elastomer by the rotor as the rotor nutates within the stator. The combined effects of interference, centrifugal forces, and sliding or rubbing of the rotor within the stator generate heat within the stator elastomer, causing the temperature of the elastomer to rise. Also, as the rotor nutates within the stator, the elastomer compresses and expands repeatedly. Heat is generated by internal viscous friction of the elastomer molecules, a phenomenon known as hysteresis. Furthermore, heat may be generated by other downhole sources. Heat from these mechanisms—interference, centrifugal forces, hysteresis, and other downhole sources—may cause the elastomer temperature to rise above its rated temperature, resulting in shortened elastomer life or its failure.
FIG. 3 illustrates a conventional rotor-stator assembly 50 that includes a rotor 55 inside a stator 60. The stator 60 further includes an elastomeric liner 62 inside an outer housing 65. This conventional rotor-stator design and others similar to it are prone to high centrifugal forces as the rotor 55 turns within the stator 60 due to the high eccentricity of the rotor-stator assembly 50. As described above, these forces generate heat causing the elastomer temperature to rise during operation of the rotor-stator assembly 50. Additionally, the elastomer design itself inhibits the ability of the elastomer 62 to dissipate heat due to the liner thickness and its relatively low thermal conductivity. Assuming all other factors remain constant, the greater the thickness of the elastomer and the lower its thermal conductivity, the greater the capacity of the elastomer to retain heat.
Attempts have been made to modify the conventional design of the stator elastomer in an effort to reduce heat retention by the elastomer. FIG. 4 illustrates a modified stator 70, referred to as a constant wall stator, comprising an elastomeric liner 75 with a reduced, as compared to elastomeric liner 62 illustrated in FIG. 3, uniform thickness inside an outer housing 80. By reducing the thickness of the elastomeric liner 75, its ability to retain heat is also reduced. However, this design modification does not directly address the sources of that heat—the centrifugal forces resulting from nutation of the rotor within the stator and the eccentricity of the rotor-stator assembly. Moreover, this design configuration adds manufacturing complexity, and therefore expense, due to the non-cylindrical inner surface or shape of the stator housing 80. Still further, this design configuration also limits the range of applications for which the housing 80 may be used. With a housing having a cylindrical inner shape or surface, the lobe configuration in the rotor-stator assembly (e.g., the number of lobes) is commonly changed simply by replacing the elastomeric liner in the stator, whereas the stator housing design illustrated in FIG. 4 is limited to the lobe configuration shown (i.e., three lobed stator configuration).
Due to the shortcomings of conventional rotor-stator assemblies described above, there remains a need for an improved rotor and stator for use in a PCP or power section of a PDM. Such an improved rotor and stator would be particularly well received if it offered the potential to reduce heat generation from centrifugal forces, heat retention by elastomeric components (e.g., the elastomeric stator liner), if present, and/or manufacturing costs while retaining design configuration flexibility.